1. Field of the Invention
This invention relates to electronic components including a plurality of substrates which are joined together via an adhesive and are arranged to have a space defined therebetween, and more particularly, to improved electronic components, for example, a piezoelectric vibrating component which has an improved structure for allowing vibration of the component to be unimpeded and for preventing separation of the substrates at adhered portions thereof.
2. Description of Related Art
Electronic components must be sealed at exterior portions thereof in order to provide a required waterproof quality and resistance to contamination from external affects and debris. In addition, piezoelectric vibrational components and similar devices must be made so as not to prevent a vibrational part from vibrating freely. Such vibrational components are usually provided with a space surrounding a vibrational portion thereof so as to allow for unimpeded vibration and to avoid any hindrance to vibration of the vibrational portion.
One example of a piezoelectric vibrational component having such a space is disclosed in Japanese Laid-open Patent Publication No. H4-4604 and Japanese Laid-open Patent Publication No. H4-35404.
FIG. 9 is a sectional view for explaining one example of a conventional piezoelectric vibrational component. A piezoelectric vibrational device 51 has an energy-trap piezoelectric vibrating element 52 having a plate shape and which utilizes thickness extensional vibration. The piezoelectric vibrating element 52 includes a piezoelectric ceramic plate 52a that is polarized in a thickness direction. The plate 52a has a resonant electrode 52b located on an upper surface thereof and a resonant electrode 52c located on a lower surface thereof so as to be opposite to the resonant electrode 52b. A resonant portion is located at a portion of the piezoelectric ceramic plate 52a which is sandwiched, in a thickness direction, by the resonant electrodes 52b, 52c. The resonant electrodes 52b, 52c extend respectively to end surfaces 52d, 52e of the piezoelectric ceramic plate 52.
The piezoelectric vibrating element 52 has a case substrate 54 adhered via an adhesive 53 onto the upper surface thereof, and a case substrate 56 adhered via an adhesive 55 onto the lower surface thereof. The case substrates 54, 56 respectively have recesses 54a, 56a defined therein. The recesses 54a, 56a are provided in order to define a space which allows the vibration portion of the plate 52a to vibrate freely and without being hindered.
The adhesives 53, 55 are provided to join the case substrates 54, 56 onto the piezoelectric vibrating element 52. If an excessive amount of either or both of the adhesives 53, 55 is used, the adhesive flows into the resonant part of the plate 52a, resulting in deterioration in resonant characteristics and hindering of the vibration of the resonant portion of the plate 52a. Even if the adhesives 54, 56 do not contact the resonant part of the plate 52a, if excessive amounts of the adhesives 53, 55 are provided on a vibration energy attenuating portion of the plate 52a located near the resonant part, deterioration in resonant characteristics occurs.
Consequently, the amount of adhesives 53, 55 applied is hoped to be sufficient for adhesion only while preventing the adhesives 53, 55 from reaching or extending along the plate 52a into the recesses 54a, 56a. However, since pressure is exerted after applying adhesives 53, 55 to achieve sufficient adhesion, the adhesive 53 and the adhesive 55 partially flow along the plate 52a into the recess 54a and into the recess 56a, respectively, and are thereafter cured. To avoid the affect of such adhesive 53, 55 in the recesses 54a, 56a, the amount of adhesive 53, 55 has been controlled such that the adhesive 53, 55 is applied only to a portion of an adhesion interface surface of the case substrates 54, 56. That is, the adhesive 53, 55 is applied in an amount and manner that the adhesive does not cover an entire adhesion interface surface but as seen in FIG. 9, only covers a portion of the substrates 54, 56 which are to be joined to the plate 52a via the adhesives 53, 55.
The piezoelectric substrate 52a constituting the piezoelectric vibrating element 52 and the case substrates 54, 56 are made of different materials and therefore, have different thermal expansion coefficients. As a result, when the piezoelectric vibrational component 51 is exposed to temperature changes, the adhered portions at locations of the adhesives 53, 55 experience stress due to the difference of thermal expansion coefficients between the material used to form the plate 52a and the material used to form the substrates 54, 56. The stresses are applied in a shearing direction as shown by the arrow A in FIG. 9. Further, the stresses concentrate at a portion shown by the arrow B, i.e. at an edge portion of an adhesion interface surface between the horizontally extending adhesion surface 54b of the case substrate 54 and the adhesive 53. This stress concentration may also occur at the end at an edge portion of an adhesion interface surface between the horizontally extending adhesion surface 56b of the case substrate 56 and the adhesive 55.
If abrupt temperature changes occur, stripping or peeling of the adhesive 53, 55 from the substrates 54, 56 and the plate 52a occurs at a stress concentration location shown by the arrow B, which results in deterioration of the waterproof quality and resistance to external factors and contamination.